80 research outputs found
Multi-line spectro-polarimetry on active region NOAA 9125
We present here some preliminary results of observations performed with the spectro-polarimetric mode of the THEMIS telescope onactiv e regionNO AA 9125 on August 2000. We show the presence of high-velocity downflows located at the edge of a filament and stress on the non-stationary character of these flows. Flaring activities were also observed: we just present here some spectro-polarimetric
profiles of these regions, more accurate magnetic-field configuration determination being performed latter on
The ground-based solar observations database BASS 2000
BASS 2000 is the French solar database for ground-based instruments. We
describe hereafter our organization, our tasks and the products we can deliver
to the international community. Our prospects cover data mining into the THeMIS
archive, a participation to the EST endeavour and the creation and curation of
the ESPaDOnS/NARVAL stellar spectra database.Comment: 3 pages, 1 figure (to appear in the Procs. of Solar Polarization
Workshop #5, eds. Berdyugina, Nagendra and Ramelli
The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO
Solar flares presumably have an impact on the deepest layers of the solar
atmosphere and yet the observational evidence for such an impact is scarce.
Using ten years of measurements of the Na D and Na D Fraunhofer
lines, measured by GOLF onboard SOHO, we show that this photospheric line is
indeed affected by flares. The effect of individual flares is hidden by solar
oscillations, but a statistical analysis based on conditional averaging reveals
a clear signature. Although GOLF can only probe one single wavelength at a
time, we show that both wings of the Na line can nevertheless be compared. The
varying line asymmetry can be interpreted as an upward plasma motion from the
lower solar atmosphere during the peak of the flare, followed by a downward
motion.Comment: 13 pages, 7 figure
Which solar EUV indices are best for reconstructing the solar EUV irradiance ?
The solar EUV irradiance is of key importance for space weather. Most of the
time, however, surrogate quantities such as EUV indices have to be used by lack
of continuous and spectrally resolved measurements of the irradiance. The
ability of such proxies to reproduce the irradiance from different solar
atmospheric layers is usually investigated by comparing patterns of temporal
correlations. We consider instead a statistical approach. The TIMED/SEE
experiment, which has been continuously operating since Feb. 2002, allows for
the first time to compare in a statistical manner the EUV spectral irradiance
to five EUV proxies: the sunspot number, the f10.7, Ca K, and Mg II indices,
and the He I equivalent width.
Using multivariate statistical methods such as multidimensional scaling, we
represent in a single graph the measure of relatedness between these indices
and various strong spectral lines. The ability of each index to reproduce the
EUV irradiance is discussed; it is shown why so few lines can be effectively
reconstructed from them. All indices exhibit comparable performance, apart from
the sunspot number, which is the least appropriate. No single index can
satisfactorily describe both the level of variability on time scales beyond 27
days, and relative changes of irradiance on shorter time scales.Comment: 6 figures, to appear in Adv. Space. Re
Automatic detection of limb prominences in 304 A EUV images
A new algorithm for automatic detection of prominences on the solar limb in 304 A EUV images is presented, and results of its application to SOHO/EIT data discussed. The detection is based on the method of moments combined with a
classifier analysis aimed at discriminating between limb prominences, active regions, and the quiet corona. This classifier analysis is based on a Support Vector Machine (SVM). Using a set of 12 moments of the radial intensity profiles, the algorithm performs well in discriminating between the above three categories of limb structures, with a misclassification rate of 7%. Pixels detected as belonging to a prominence are then used as starting point to reconstruct the whole prominence by morphological image processing techniques. It is planned that a catalogue of limb prominences identified in SOHO and STEREO data using this method will be made publicly available to the scientific community
Seismic Emissions from a Highly Impulsive M6.7 Solar Flare
On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive
solar flare in close proximity to the solar limb. This flare has previously
been studied in great detail, with observations classifying it as a type 1
white-light flare with a very hard spectrum in hard X-rays. The flare was also
associated with a type II radio burst and coronal mass ejection. The flare
emission characteristics appeared to closely correspond with previous instances
of seismic emission from acoustically active flares. Using standard local
helioseismic methods, we identified the seismic signatures produced by the
flare that, to date, is the least energetic (in soft X-rays) of the flares
known to have generated a detectable acoustic transient. Holographic analysis
of the flare shows a compact acoustic source strongly correlated with the
impulsive hard X-ray, visible continuum, and radio emission. Time-distance
diagrams of the seismic waves emanating from the flare region also show faint
signatures, mainly in the eastern sector of the active region. The strong
spatial coincidence between the seismic source and the impulsive visible
continuum emission reinforces the theory that a substantial component of the
seismic emission seen is a result of sudden heating of the low photosphere
associated with the observed visible continuum emission. Furthermore, the
low-altitude magnetic loop structure inferred from potential--field
extrapolations in the flaring region suggests that there is a significant
inverse correlation between the seismicity of a flare and the height of the
magnetic loops that conduct the particle beams from the corona.Comment: 16 pages, 7 figures, Solar Physics Topical Issue: SOHO 19/GONG 2007
"Seismology of Magnetic Activity", Accepte
Polarization Diagnostics for Cool Core Cluster Emission Lines
The nature of the interaction between low-excitation gas filaments at ~104 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~107 K in the centers of galaxy clusters remains a puzzle. The presence of a strong, empirical correlation between the two gas phases is indicative of a fundamental relationship between them, though as yet of undetermined cause. The cooler filaments, originally thought to have condensed from the hot gas, could also arise from a merger or the disturbance of cool circumnuclear gas by nuclear activity. Here, we have searched for intrinsic line emission polarization in cool core galaxy clusters as a diagnostic of fundamental transport processes. Drawing on developments in solar astrophysics, direct energetic particle impact induced polarization holds the promise to definitively determine the role of collisional processes such as thermal conduction in the ISM physics of galaxy clusters, while providing insight into other highly anisotropic excitation mechanisms such as shocks, intense radiation fields, and suprathermal particles. Under certain physical conditions, theoretical calculations predict of the order of 10% polarization. Our observations of the filaments in four nearby cool core clusters place stringent upper limits ( 0.1%) on the presence of emission line polarization, requiring that if thermal conduction is operative, the thermal gradients are not in the saturated regime. This limit is consistent with theoretical models of the thermal structure of filament interfacesPeer reviewe
Hydrogen H line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer
Context. We present a theoretical review of the effect of impact polarization
of a hydrogen H line due to an expected proton beam bombardment in
solar flares. Aims. Several observations indicate the presence of the linear
polarization of the hydrogen H line observed near the solar limb above
5% and preferentially in the radial direction. We theoretically review the
problem of deceleration of the beam originating in the coronal reconnection
site due to its interaction with the chromospheric plasma, and describe the
formalism of the density matrix used in our description of the atomic processes
and the treatment of collisional rates. Methods. We solve the self-consistent
NLTE radiation transfer problem for the particular semiempirical chromosphere
models for both intensity and linear polarization components of the radiation
field. Results. In contrast to recent calculations, our results show that the
energy distribution of the proton beam at H formation levels and
depolarizing collisions by background electrons and protons cause a significant
reduction of the effect below 0.1%. The radiation transfer solution shows that
tangential resonance-scattering polarization dominates over the impact
polarization effect in all considered models. Conclusions. In the models
studied, proton beams are unlikely to be a satisfying explanation for the
observed linear polarization of the H line.Comment: 11 pages, 11 figures, accepted for publication in A&
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